The overall theme of the proposed research is to understand how regulation of protein stability is used to regulate growth of normal and cancer cells. We study ubiquitin (Ub)-mediated destruction of the oncoprotein Myc, a transcription factor that features prominently in the regulation of cell growth and in cancer. We have previously identified a number of elements in Myc that regulate its destruction, and we have shown that cancer-associated and transforming mutations within at least one of these elements stabilizes the Myc protein, suggesting that enhanced protein stability is a mechanism of Myc activation in cancer. Our objective for this research proposal is to perform a complete and detailed analysis of the mechanisms regulating Myc protein stability, and to apply this understanding to address how protein turnover impacts the many functions of the Myc protein. We will take a three-pronged approach to meeting this objective. First, we will further characterize the elements in Myc that regulate its destruction probing the nature of these elements, the way in which they are regulated, and how they interact to regulate Myc destruction. Second, we will characterize the cellular machinery that targets Myc for destruction, identifying Ub-ligases and other molecules that control Myc stability. Finally, we will resolve how the proteolysis of Myc is used to regulate its activity, asking how the elements in Myc that control its stability-and the proteins with which these elements interact-affect the ability of Myc to activate transcription, stimulate proliferation, and drive cellular transformation. Results of these studies will provide an important paradigm for understanding how proteolysis is used to regulate protein function and will reveal new insight into the regulation and function of a major human oncoprotein.